The present invention relates to a fuzzy feedback controller or method which, in a process control, determines a control output by fuzzy inference so that a process value coincides with an aimed value, and causes an operating device to control an object which is to be controlled.
Heretofore, in a process control, a PID (proportion, integration, and differentiation) control is generally employed. On the other hand, recently, in view of the fact that the PID control becomes unstable if control parameters are not suitably adjusted, a fuzzy feedback control of control parameter setting type has been proposed in the art in which, instead of the PI control, the fuzzy control is employed, and rules are added for prevention of the occurrence of unstable phenomena (cf. "Feedback Control Method by Fuzzy Rules", pp. 541 and 542, a manuscript for the 30-th Automatic Control Lecture Meeting).
The PID control is disadvantageous in the following points: That is, if it is intended to improve the quick response at the time of rise, then the overshoot is increased; and when it is intended to improve the attenuation in the period of stabilization, then the rise is delayed.
Hence, in order to realize both the satisfactory quick response and the attenuation, when the control characteristics is switched separately according to whether it is the time of rise or whether it is the time of stabilization, it is necessary to change a plurality of control parameters such as proportion gain, integration time, and differentiation time during control. Furthermore, when the process is changed in characteristic, the control is greatly lowered in performance if the control parameters are not suitably adjusted.
On the other hand, in the conventional fuzzy feedback control, membership functions are fixed for a given process, and therefore whenever it is required to change the control parameters, the membership functions in memory in the controller must be changed. Furthermore, in practice, the control parameters must be studied according to the contents of a given process, before set in the controller. This operation is rather troublesome.